Exceptions to Mendel’s Laws of Inheritance MENDEL’S LAWS HOLD TRUE FOR SIMPLE INHERITANCE NOW WE HAVE DISCOVERED MANY TYPES OF COMPLEX INHERITANCE WHICH INVOLVE EXCEPTIONS TO HIS RULES

Exceptions to Law One The Law of Dominance says that (1) every trait is controlled by two genes and that (2) the genes can be either dominant or recessive. Guess what!? Some traits are controlled by more than two genes! And some genes are neither dominant nor recessive! Examples include: codominance, incomplete dominance, gene interaction, and multiple alleles.

1. Incomplete Dominance Some traits are blends of genes, instead of being either all dominant or all recessive The heterozygous phenotype is a mix of the two different alleles (instead of showing the dominant phenotype only)

Incomplete Dominance, continued Let’s say that there are two genes for a trait – black hair or white hair. In simple inheritance, one color will be dominant over the other. Every creature will have either black or white hair – there is no such thing as gray . Hybrids will have the dominant color. In complex inheritance, both colors share their dominance! Some creatures will be either black or white – but others will be gray! Hybrids will always be gray, while the black and white critters will always be homozygous.

Examples of Incomplete Dominance Human hair texture: Curly hair is homozygous dominant (H1H1) Straight hair is also homozygous dominant! (H2H2) The hybrid of these (H1H2) gets wavy hair! Carnation Color: Red flowers are homozygous dominant (F1F1) White flowers are also homozygous dominant! (F2F2) The hybrid of these (F1F2) gets pink flowers!

Incomplete Dominance Punnettification! Find the phenotypes of the offspring of two wavy-haired parents! Mommy Daddy Phenotype Genotype Gametes Wavy Wavy Final Answer: 25% Curly Hair 25% Straight Hair 50% Wavy Hair H1H2 H1H2 H1 or H2 H1 or H2 H1 H2 H1 H1H1 H1H2 H2 H1H2 H2H2

2. Codominance Instead of the trait being expressed as a mix of the two genes, they are both displayed the same amount at the same time! Both alleles are expressed equally Examples Sickle Cell Anemia in Humans Human Blood Types Roan cattle Erminette Chickens (Black + White = Speckled! This is COdominance, while INComplete dominance would be gray)

Patchy coloration was not caused by a mistake at the salon! Roan Cattle Red Hair is Codominant with white hair If a homozygous white cow and a homozygous red cow breed, their offspring is referred to as a “roan”. Phenotype is 50% red hair, 50% white hair (not pink!!) Patchy coloration was not caused by a mistake at the salon!

Sickle Cell Anemia Lethal blood disorder Normal red blood cell Lethal blood disorder Defective protein changes the shape of the hemoglobin molecule Causes deformations in red blood cells – they become sickle shaped (like a crescent moon) “Sickled” red blood cell HA = normal allele Hs = sickle cell allele Alleles HAHA = normal HAHs= carrier, not affected but resistant to malaria HSHS= sufferer Genos & Phenos

Carriers A carrier is someone who has a recessive gene for a trait that is masked by a dominant gene. While they “carry” the gene, they do not suffer any symptoms or exhibit the trait. They can, however, pass on this recessive trait to their offspring With sickle cell anemia, the carrier has the advantage of being resistant to malaria We would expect the sickle cell gene to disappear over time, since it kills people before they can pass it on to their offspring. But since carriers can survive malaria, the hybrid genotype is common in people from tropical Africa and their descendants 10

Sickle Cell Anemia What are the chances of a carrier female and a normal male having a normal child? Do the Punnett Square! Check your work here in 3 minutes! Female genotype = HAHs Male genotype = HAHA HA Hs HAHA HAHs Final Answer: 50% Chance of this couple having a normal child! 11

3. Gene Interaction Some traits are controlled by more than one pair of genes – the genes might not even be on the same chromosome! This is referred to as a “polygenic” trait This allows for a wide range of possible phenotypes Examples: Eye color Skin color

4. Multiple Alleles Some traits have more than two possible alleles Example: Blood types 3 Possible alleles for blood type: A, B, & O Codominance makes 4 possible phenotypes: A (Dominant allele) B (Also Dominant) AB (Codominant) O (Recessive allele)

Go to Worksheet…. 1. Get “Human Genotypes and Phenotypes” worksheet 2. Read the intro 3. Use the descriptions and pictures on the front to help you fill in the chart on the back 4. For the column labeled “Your Phenotype,” write down your physical traits (the way you look). Examples: “Widow’s Peak,” or “Round Eyes” 5. For the column labeled “Your Genotype,” write down your possible genetic code, using the letters from the front page. Examples: if you have a Widow’s Peak, you need to write down two possible genotypes (WW or Ww). You need to include both, since you don’t know which you are for sure. If you have Round Eyes, only write down aa, since it is the only possible genotype for you. 6. Answer all the questions. 7. Start homework: 7.2 Vocab AND questions

Exceptions to Law 2 The Law of Segregation says that the two alleles for a trait separate during meiosis, so that a new organism only gets one allele from each parent. Sometimes mistakes occur where a sperm or egg cell has an extra or a missing chromosome – this is called nondisjunction: the chromosomes did not separate properly Examples include trisomy (3 chromosomes) and monosomy (1 chromosome) disorders, such as Down’s Syndrome, Turner’s Syndrome, and Klinefelter’s Syndrome – details on another powerpoint!

Exceptions to Law 3 The Law of Independent Assortment says that each pair of alleles separates independently from every other pair – in other words, the pairs are sorted randomly. However, many traits are found on one chromosome (like 10,000 genes per chromosome!). This means that these traits are inherited as a group, and they are not really independent. Examples include Gene Linkage and Crossing Over

Gene Linkage Many traits are on one chromosome, so these traits are not independent, they change together Examples include sex-linked traits, where genes are found on the X chromosome The sex-linked traits have nothing to do with gender, since both males and females have an X chromosome

Sex-Linked Traits Females get two versions of genes A, B, and C, so they can be hybrids and mask a recessive trait Males only get one version of genes A, B, and C, so they will show the recessive trait even if they only inherited one copy of this gene This means sex-linked recessive traits are more common in males than females, since females can mask the trait if they are hybrids Examples include hemophilia and color blindness More details on another powerpoint!

Exception to the Exception of Law 3!! Crossing Over: Homologous chromosomes sometimes trade segments during meiosis, which breaks the gene linkage and makes the traits independent again! This increases the genetic variation in the gene pool, which can help the species survive if their environment changes

Vocab Match Having more than one trait on a chromosome. Blending of alleles in hybrids. Having three chromosomes instead of a pair. Multiple pairs of genes influencing the same trait. More than two versions of a gene for a trait. Chromosomes trading segments during meiosis. Two alleles equally expressed for the same trait. Failure of chromosomes to separate during meiosis. Traits controlled by alleles on the X chromosome. Having only one chromosome instead of a pair.

Having more than one trait on a chromosome = Gene Linkage Blending of alleles in hybrids = Incomplete Dominance Having three chromosomes instead of a pair = Trisomy Multiple pairs of genes influencing the same trait = Polygenic Inheritance or Gene Interaction More than two versions of a gene for a trait = Multiple Alleles Chromosomes trading segments during meiosis = Crossing Over Two alleles equally expressed for the same trait = Codominance Failure of chromosomes to separate during meiosis = Nondisjunction Traits controlled by alleles on the X chromosome = Sex-Linked Traits Having only one chromosome instead of a pair = Monosomy